This invention relates to digital communication networks; and, more particularly, it relates to methods of encoding and serially transmitting sequences of self-clocking data characters and control characters from one communication node to another in a local area network.
Briefly, a local area network is comprised of a plurality of communication nodes which are serially interconnected in a loop. Each node can be any type of data processing equipment, such as a disc, a printer, a video display or a computer. Data messages travel around the loop from one communication node to another until they pass through all nodes on the network, and an address field specifies which of the nodes the data message is for.
Each data message is made up of a sequence of data characters; and each data character is made of a group of zero and one bits. Typically there are eight bits per character. Data characters are characters which are printed out by a printer, displayed on a screen by a video display, or stored for later retrieval in a disc.
But in addition to the data characters, control characters are also sent around the loop. One function of the control characters is that they format the data messages. For example, one control character may indicate where a data message begins; another control character may indicate where a data message ends; and still another control character may simply indicate that no data message is on the network,--i.e., that the network is in an idle mode. Another function of the control characters is to give commands or instructions to a node. For example, one control character may direct a node to enter a diagnostic mode of operation, another control character may direct a node to turn off, etc.
A common prior art method of encoding and serially transmitting data and control characters on a local area network is the F/2F encoding method. With the F/2F method, a zero is transmitted as a signal transition at the beginning and end of a bit period; and a one is transmitted as a signal transition at the beginning, middle, and end of a bit period.
Control characters in the F/2F method are transmitted as characters which contain violations of the above zero and one signal transitions. For example, the omission of a signal transition at the end of the first bit period of a character may indicate one control character; the omission of a signal transition at the end of the second bit period of a character may indicate another control character, etc.
A problem, however, with generating control characters as violations of normal F/2F encoding is that removing transitions from the edge of bits in the transmitted signal makes synchronizing a phase-locked oscillator to the signal quite difficult. Another problem with generating control characters as F/2F code violations is that the number of different characters that can be generated is quite limited. Yet another problem is that generating and receiving control characters as F/2F violations is very cumbersome since they have non-one and nonzero bits; and logic circuits only operate with ones and zeros. Also, a problem with F/2F encoding in general is that the encoded data requires a high bandwidth for transmissions.
Accordingly, a primary object of the invention is to provide an improved method of encoding and serially transmitting self-clocking data characters and control characters which avoids the above prior art problems.